This invention relates to a cleaning method for a filter, and a filter system, and more particularly to a filter which is best suited for use in a filter system for removing cruds (corrosion products) in a nuclear plant by relieving the discs from an assembled or piled condition in a filter for facilitating reverse cleaning.
Description will be given with reference to a filter, in which a plurality of discs having grooved surfaces are assembled or piled one on top of another, so that fluid to be processed is introduced into channels or grooves thus prepared for removing impurities. This type filter is referred to as an etched disc filter, hereinafter.
The prior art etched disc filters suffer from the short-comings as follows:
The discs are assembled one on top of another in press-contacting relation, so that upon reverse cleaning, the fluid should pass through flow passages of a extremely small cross sectional area, which are prepared according to etching, thus leading to a failure to achieve a desired reverse-cleaning effect. In a nuclear plant, cruds filtered and separated tend to adhere to the surfaces of the filter, because of a large tackiness of cruds and cohesion forces of cruds, with the result that cruds can not be separated completely by resorting only to a pressurized flow of a fluid through the passages. As a result, even if the reverse cleaning is applied, a pressure loss fails to recover to its initial condition.
Meanwhile, in case an etched disc filter is used for filtering suspension existing in a general type waste liquid, the discs may be taken out or disassembled with ease for cleaning. However, in case a radioactive material such as cruds are to be filtered, the discs can not be taken out.
For cleaning discs, there may be proposed attempts such as the reverse flowing of a cleaning liquid, for instance, citric acid or diluted hydrochloric acid and the like, or a cleaning device which is provided in a filter tank. These pose problems of complicated operations in maintenance. The etched disc filter is now under consideration for use in a nuclear plant as a crud filtering or removing device, because of its excellent filtering performance, although the proposed use of etched disc filter has to be confronted by the aforesaid problems arising from a maintenance aspect.